"Caffeine inhibits the contractions of the muscles in the fallopian tube, so the egg stops getting transported," lead researcher Sean Ward of the University of Nevada School of Medicine tells MSNBC.

If the eggs aren't delivered to the womb, a woman will either have an unsuccessful pregnancy or be at risk for an ectopic pregnancy (when the embryo gets stuck and develops in the woman's fallopian tube), he adds.

MSNBC reports researchers studied the effect of caffeine on the fallopian tubes of mice, who have reproductive systems similar to humans. (That explains why Mickey and Minnie Mouse have been together for 83 years but don't have any children.)

Ward tells MSNBC the mice got the amount of caffeine equivalent to a couple of cups of coffee. Still, much more research is needed to determine how much caffeine is necessary to impair fertility in humans, he adds.